1. Field of the Invention
The present invention relates to a method and apparatus for controlling pyrolytic cleaning in a combination microwave and convection heating oven in which blower actuation, such as jet impingement actuation, is deferred to a time or an event near the terminal portion of the cleaning cycle.
2. Background Art
Many cooking ovens have a pyrolytic cleaning function in which the heating elements are operated to obtain a pyrolytic cleaning temperature, for example 800xc2x0 F. (427xc2x0 C.) within the oven chamber. Typically, the oven is sealed so that accumulated debris on the walls of the oven can be incinerated and removed as ash residue after the pyrolytic cleaning cycle has terminated. The operation of such cleaning cycles is regulated and test results must comply with standards established, for example Underwriters Laboratory (UL) 858 standard for household electric ranges.
Although recent improvements of cooking devices have employed multiple cooking energy sources such as microwave generators, convection heating sources, and conventional radiant heating elements, the control of pyrolytic cleaning cycles is limited to actuation of radiant heating elements which can be powered to achieve the high temperatures in the oven that are required for pyrolytic cleaning. The operation of additional heating sources during a cleaning cycle is problematic.
One previously known oven construction included a passageway surrounding the oven chamber in which a suction fan operates to cool the outer housing of the cooking appliance. The passageway communicates with the interior oven chamber through a throttled passageway to provide exhaustion of gases emanating from food being processed in the oven chamber. The throttled opening is controlled by a thermally responsive gate so that the throttle passageway is closed during pyrolytic heating of the oven chamber to avoid the release of volatile gases from the oven chamber during cleaning. Accordingly, pyrolytic cleaning is limited to the interior oven chamber.
Another combination microwave and electric self-cleaning oven is disclosed in U.S. Pat. No. 4,547,642. This combination oven includes an air vent from the cooking cavity communicating with an air exhaust passageway including a catalytic oxidation unit that decomposes gases generated when the oven is operating in a self-cleaning mode. A restricted air inlet is configured to limit unaugmented airflow into the oven chamber during the self-cleaning mode. A humidity sensor controls the operation of a blower that augments the flow of air into the cavity when the humidity level caused by microwave only heating is used for cooking.
More recent developments in cooking appliances have combined jet impingement heating sources with microwave cooking sources and radiant heat elements to additionally speed up and control the cooking of foods within the oven chamber. However, while such combined cooking sources accelerate the time of cooking processes, such combinations have not been useful in pyrolytic cleaning operations of those appliances within the test standards of regulations previously discussed.
The present invention overcomes the above mentioned disadvantages by providing a combination cooking source oven having a control that generates a multiple stage pyrolytic cleaning operation. The present invention provides a method and apparatus for pyrolytically cleaning an oven chamber by sealing the chamber, raising the temperature to a pyrolytic cleaning level for a first operating period in which the blower operation is disabled, maintaining the temperature of the pyrolytic cleaning level and actuating the blower operation in a second time period following the first operating period.
In the preferred embodiment, a blower of a jet impingement cooking source is disabled while up to four heating elements are operated to obtain a pyrolytic cleaning temperature within a sealed oven chamber. An additional stage is initiated by the oven control to generate jet impingement currents throughout the oven chamber and the passageways in communication with the chamber that may have been soiled during cooking operations. Preferably, the pyrolytic cleaning temperature is maintained in the second stage while the risk of volatile gases and their inadvertent ignition is avoided by consumption of the gases in the initial cleaning stage. The duration of each stage may be fixed or variable as desired. The preferred embodiment of the method is preferably open loop, and relies only on a fixed time duration for each stage to simplify the control. However, the invention includes closed loop systems, wherein the initiation of the second stage is triggered by an event, such as a sensor detection that enables the control to determine when a level of volatile gases or oxygen level has been sufficiently reduced, so as to avoid excessive operation of the heaters in the cleaning stage.
As a result, the present invention provides a combination cooking source oven with a self-cleaning operation that complies with regulatory guidelines for pyrolytic cleaning operation. Unlike previous controls that operate in response to cooking conditions occurring within the oven, the present invention provides a pyrolytic cleaning operation that effectively cleans additional surfaces in communication with the oven chamber that may be subjected to the accumulation of debris during cooking.